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class="title">Sax support</h1>
<p>In this document we'll describe lxml's SAX support. lxml has support for
producing SAX events for an ElementTree or Element. lxml can also turn SAX
events into an ElementTree. The SAX API used by lxml is compatible with that
in the Python core (xml.sax), so is useful for interfacing lxml with code that
uses the Python core SAX facilities.</p>
<div class="contents topic" id="contents">
<p class="topic-title first">Contents</p>
<ul class="simple">
<li><a class="reference internal" href="#building-a-tree-from-sax-events" id="id1">Building a tree from SAX events</a></li>
<li><a class="reference internal" href="#producing-sax-events-from-an-elementtree-or-element" id="id2">Producing SAX events from an ElementTree or Element</a></li>
<li><a class="reference internal" href="#interfacing-with-pulldom-minidom" id="id3">Interfacing with pulldom/minidom</a></li>
</ul>
</div>
<div class="section" id="building-a-tree-from-sax-events">
<h1>Building a tree from SAX events</h1>
<p>First of all, lxml has support for building a new tree given SAX events. To
do this, we use the special SAX content handler defined by lxml named
<tt class="docutils literal">lxml.sax.ElementTreeContentHandler</tt>:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">lxml.sax</span>
<span class="gp">>>> </span><span class="n">handler</span> <span class="o">=</span> <span class="n">lxml</span><span class="o">.</span><span class="n">sax</span><span class="o">.</span><span class="n">ElementTreeContentHandler</span><span class="p">()</span>
</pre></div>
<p>Now let's fire some SAX events at it:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="n">handler</span><span class="o">.</span><span class="n">startElementNS</span><span class="p">((</span><span class="bp">None</span><span class="p">,</span> <span class="s1">'a'</span><span class="p">),</span> <span class="s1">'a'</span><span class="p">,</span> <span class="p">{})</span>
<span class="gp">>>> </span><span class="n">handler</span><span class="o">.</span><span class="n">startElementNS</span><span class="p">((</span><span class="bp">None</span><span class="p">,</span> <span class="s1">'b'</span><span class="p">),</span> <span class="s1">'b'</span><span class="p">,</span> <span class="p">{(</span><span class="bp">None</span><span class="p">,</span> <span class="s1">'foo'</span><span class="p">):</span> <span class="s1">'bar'</span><span class="p">})</span>
<span class="gp">>>> </span><span class="n">handler</span><span class="o">.</span><span class="n">characters</span><span class="p">(</span><span class="s1">'Hello world'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">handler</span><span class="o">.</span><span class="n">endElementNS</span><span class="p">((</span><span class="bp">None</span><span class="p">,</span> <span class="s1">'b'</span><span class="p">),</span> <span class="s1">'b'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">handler</span><span class="o">.</span><span class="n">endElementNS</span><span class="p">((</span><span class="bp">None</span><span class="p">,</span> <span class="s1">'a'</span><span class="p">),</span> <span class="s1">'a'</span><span class="p">)</span>
</pre></div>
<p>This constructs an equivalent tree. You can access it through the <tt class="docutils literal">etree</tt>
property of the handler:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="n">tree</span> <span class="o">=</span> <span class="n">handler</span><span class="o">.</span><span class="n">etree</span>
<span class="gp">>>> </span><span class="n">lxml</span><span class="o">.</span><span class="n">etree</span><span class="o">.</span><span class="n">tostring</span><span class="p">(</span><span class="n">tree</span><span class="o">.</span><span class="n">getroot</span><span class="p">())</span>
<span class="go">b'<a><b foo="bar">Hello world</b></a>'</span>
</pre></div>
<p>By passing a <tt class="docutils literal">makeelement</tt> function the constructor of
<tt class="docutils literal">ElementTreeContentHandler</tt>, e.g. the one of a parser you configured, you
can determine which element class lookup scheme should be used.</p>
</div>
<div class="section" id="producing-sax-events-from-an-elementtree-or-element">
<h1>Producing SAX events from an ElementTree or Element</h1>
<p>Let's make a tree we can generate SAX events for:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="n">f</span> <span class="o">=</span> <span class="n">StringIO</span><span class="p">(</span><span class="s1">'<a><b>Text</b></a>'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">tree</span> <span class="o">=</span> <span class="n">lxml</span><span class="o">.</span><span class="n">etree</span><span class="o">.</span><span class="n">parse</span><span class="p">(</span><span class="n">f</span><span class="p">)</span>
</pre></div>
<p>To see whether the correct SAX events are produced, we'll write a custom
content handler.:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">xml.sax.handler</span> <span class="kn">import</span> <span class="n">ContentHandler</span>
<span class="gp">>>> </span><span class="k">class</span> <span class="nc">MyContentHandler</span><span class="p">(</span><span class="n">ContentHandler</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>
<span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">a_amount</span> <span class="o">=</span> <span class="mi">0</span>
<span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">b_amount</span> <span class="o">=</span> <span class="mi">0</span>
<span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">text</span> <span class="o">=</span> <span class="bp">None</span>
<span class="gp">...</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">startElementNS</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">name</span><span class="p">,</span> <span class="n">qname</span><span class="p">,</span> <span class="n">attributes</span><span class="p">):</span>
<span class="gp">... </span> <span class="n">uri</span><span class="p">,</span> <span class="n">localname</span> <span class="o">=</span> <span class="n">name</span>
<span class="gp">... </span> <span class="k">if</span> <span class="n">localname</span> <span class="o">==</span> <span class="s1">'a'</span><span class="p">:</span>
<span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">a_amount</span> <span class="o">+=</span> <span class="mi">1</span>
<span class="gp">... </span> <span class="k">if</span> <span class="n">localname</span> <span class="o">==</span> <span class="s1">'b'</span><span class="p">:</span>
<span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">b_amount</span> <span class="o">+=</span> <span class="mi">1</span>
<span class="gp">...</span>
<span class="gp">... </span> <span class="k">def</span> <span class="nf">characters</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">data</span><span class="p">):</span>
<span class="gp">... </span> <span class="bp">self</span><span class="o">.</span><span class="n">text</span> <span class="o">=</span> <span class="n">data</span>
</pre></div>
<p>Note that it only defines the startElementNS() method and not startElement().
The SAX event generator in lxml.sax currently only supports namespace-aware
processing.</p>
<p>To test the content handler, we can produce SAX events from the tree:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="n">handler</span> <span class="o">=</span> <span class="n">MyContentHandler</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">lxml</span><span class="o">.</span><span class="n">sax</span><span class="o">.</span><span class="n">saxify</span><span class="p">(</span><span class="n">tree</span><span class="p">,</span> <span class="n">handler</span><span class="p">)</span>
</pre></div>
<p>This is what we expect:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="n">handler</span><span class="o">.</span><span class="n">a_amount</span>
<span class="go">1</span>
<span class="gp">>>> </span><span class="n">handler</span><span class="o">.</span><span class="n">b_amount</span>
<span class="go">1</span>
<span class="gp">>>> </span><span class="n">handler</span><span class="o">.</span><span class="n">text</span>
<span class="go">'Text'</span>
</pre></div>
</div>
<div class="section" id="interfacing-with-pulldom-minidom">
<h1>Interfacing with pulldom/minidom</h1>
<p>lxml.sax is a simple way to interface with the standard XML support in the
Python library. Note, however, that this is a one-way solution, as Python's
DOM implementation cannot generate SAX events from a DOM tree.</p>
<p>You can use xml.dom.pulldom to build a minidom from lxml:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="kn">from</span> <span class="nn">xml.dom.pulldom</span> <span class="kn">import</span> <span class="n">SAX2DOM</span>
<span class="gp">>>> </span><span class="n">handler</span> <span class="o">=</span> <span class="n">SAX2DOM</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">lxml</span><span class="o">.</span><span class="n">sax</span><span class="o">.</span><span class="n">saxify</span><span class="p">(</span><span class="n">tree</span><span class="p">,</span> <span class="n">handler</span><span class="p">)</span>
</pre></div>
<p>PullDOM makes the result available through the <tt class="docutils literal">document</tt> attribute:</p>
<div class="syntax"><pre><span class="gp">>>> </span><span class="n">dom</span> <span class="o">=</span> <span class="n">handler</span><span class="o">.</span><span class="n">document</span>
<span class="gp">>>> </span><span class="k">print</span><span class="p">(</span><span class="n">dom</span><span class="o">.</span><span class="n">firstChild</span><span class="o">.</span><span class="n">localName</span><span class="p">)</span>
<span class="go">a</span>
</pre></div>
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